In the measurement or calibration of a capacitor, traceability (values transfer) of capacitance parameters requires a high-precision capacitance bridge, or a high-precision single-value standard capacitor, or a high-precision capacitance standard box. Currently, the uncertainty of the Capacitance Standard Box is generally at 1%˜0.05%; its capacitance range is within 1 pF˜1 μF, and its zero capacitance is ≦0.005 pF. The uncertainty of the relatively good capacitance standard box can reach 0.05%. From the viewpoint of development and demand, the currently existing capacitance standard box with the uncertainty of 0.05% cannot meet the requirements of higher precision in practical application.
The capacitance standard box may have different decimal digits according to different output ranges. In addition that the connected standard capacitor at each decimal digit has different capacitance units, the decimal digit structures are the same. The capacitance standard box comprises a high-precision capacitor (or called as a standard capacitor), a changeover switch, an input terminal, an output terminal and a shield box. The uncertainty of the capacitance output from the capacitance standard box is mainly determined by the uncertainty of the standard capacitors and the changeover switch. According to the current technology, the uncertainty of the changeover switch is generally within 0.1%˜0.01%. Plus the uncertainty of the standard capacitor itself and the uncertainty introduced from leading wires and an interface, the uncertainty of the output capacitance of the currently existing capacitance standard box cannot meet the requirement of 0.01% uncertainty.
The currently existing capacitance standard box with 1%˜0.05% uncertainty used a decimal changeover switch and ten capacitors with the same capacitance. The different number of standard capacitors is selected to be connected in parallel through a changeover switch. Total 11 values of 0, 1, 2, . . . up to 10 capacitance values can be made up. For example, ten high-precision capacitors in 100 pF (this 100 pF is the unit-capacitance of this decimal. Please refer to the (C1˜C10) in Table 1) are used as input; 11 output values are within 0-10 unit-capacitances; that is (0˜10)×100 pF capacitances, and the error introduced by the changeover switch is within 0.1%˜0.01%. FIG. 1 is a circuit diagram of a decimal of a capacitance standard box. The figure shows a circuit diagram, in which a changeover switch selected standard capacitances C=C1+C2+C3+C4 as output, and the capacitance is 4 of unit capacitance. The output capacitance between A and A′ is 400 pF=4×100 pF. The output of any other capacitance from 0 to 1000 pF can be selected in a similar way. The selection between the output capacitance and the changeover switch is shown in Table 1.
In a capacitance standard box with the output range of 1˜1000 pF, the unit capacitance of the first digit is 100 pF; the unit capacitance of the second digit is 10 pF, and the unit capacitance of the third decimal is 1 pF. If a standard capacitance of 893 pF is needed, the first digit of the capacitance standard box output is eight unit capacitances; its second digit output is nine unit capacitances, and its third digit output is three unit capacitances. Other output values can be made in the same way.
In the currently existing capacitance standard box, the main reasons of the relatively large error caused by the changeover switch are as follows:
1. As ten standard capacitors are needed to meet the requirements of eleven standard capacitance values, the structure of a changeover switch is complex; the distributed capacitance is relatively high; more leading wires at the input terminal is needed; the number of leading wires is 20; the distributed capacitance is introduced; terminals are not completely shielded, and hence error is introduced.
2. As a shield board between electrodes (a partition metal board in a shield box) is passed through by ten input wires, two relatively large holes are opened on the shield board, thereby causing the distributed capacitance between the electrodes increased, and hence introducing error.
3. As an output wire of the changeover switch does not use a shielded wire but a plain wire (a silver-plated copper wire); furthermore, two output interfaces are not completely shielded, and the distributed capacitance is introduced, the error is introduced.
4. As the shield box is made from a bolted metal sheet, which is poor shield to external electrical field and magnetic field, this will add in error.
Out of all above, first three are the main error sources, which are inherent in the structure of currently existing capacitance standard box.